1. Field of the Invention
The present invention relates to a magnesium-based alloy for high temperature and a manufacturing method thereof.
2. Description of the Prior Art
Magnesium with a specific gravity of 1.7 is not only the lightest element among commercially available metals, but its specific strength and specific stiffness are also superior to those of iron and aluminum. In addition, excellent mechanical properties can be obtained when manufacturing magnesium products by a die casting process. Therefore, magnesium is currently being applied to various fields, such as portable electronic components, aircrafts and sporting goods, etc., with mainly focusing on the field of automobile components. When magnesium alloys are applied to the automobile components, 30% of a weight reduction can be achieved.
Typical magnesium alloys among the currently available commercial magnesium alloys for die casting applications are magnesium (Mg)-aluminum (Al) based alloys such as AZ91D, AM50 and AM60. These magnesium alloys are low priced and have good castability as compared to other alloys for the die casting applications. Particularly, high strength can be obtained by forming a β-Mg17Al12 phase during solidification at room temperature. However, while automobile and aircraft components are generally used in a high temperature environment of 150-200° C., poor thermal stability of the β phase deteriorates creep resistance of these magnesium alloys. As a result, there is a disadvantage that these magnesium alloys are not appropriate for applying to the foregoing products used in the high temperature environment.
Many efforts have been made to develop and optimize magnesium alloys for high temperature since the 1990's. Magnesium alloys for high temperature are largely classified into magnesium alloys for die casting applications and magnesium alloys for sand casting applications, which depend on alloy compositions and manufacturing methods caused by differences in use temperatures of target components. A proper characteristic required for the magnesium alloy for high temperature is castability that is appropriate for die casting, and corrosion and oxidation resistances are also required. In addition, when considering competitiveness against steel and aluminum, development of magnesium alloys excluding high-priced additive elements is required in terms of cost.
When examining conventionally developed magnesium alloys for high temperature based on the above requirements, magnesium alloys having high addition ratios of rare earth elements (RE) are disadvantageous in an aspect of cost. On the other hand, when adding alkaline earth metals (e.g., calcium (Ca) and strontium (Sr)) into magnesium alloys, there is a problem that the magnesium alloys have poor castability such as decrease in melt fluidity, hot tear cracks, and die soldering.